Polysiloxane pressure-sensitive adhesive compositions have excellent electrical insulating properties, heat resistance, cold resistance, and pressure-sensitive adhesion with regard to various adherends as compared to an acrylic or rubber pressure-sensitive adhesive compositions, and therefore are used in heat resistant pressure-sensitive adhesive tape, electrical insulating pressure-sensitive adhesive tape, heat sealing tape, plating masking tape, and the like. The polysiloxane pressure-sensitive adhesive compositions are categorized into addition reaction curing types, condensation reaction curing types, peroxide curing types, and the like, based on the curing mechanism thereof. The addition reaction curing type pressure-sensitive adhesive compositions are widely used, because curing is quickly performed at room temperature or by heating, and byproducts are not generated.
In recent years, application to the field of electronic display elements such as smart devices and the like has been studied, utilizing the aforementioned properties and high transparency of the polysiloxane pressure-sensitive adhesives. The device has a structure where a film formed from a plurality of layers including an electrode layer and display layer is interposed between transparent substrates, and in order to protect the electrode layer and display layer and improve adhesion between the layers, a polysiloxane pressure-sensitive adhesive with high heat resistance, cold resistance, and transparency is expected to function effectively.
In these smart apparatuses, high dielectric properties are included in addition to excellent transparency as material properties of the pressure-sensitive adhesive required in pressure sensors and other sensor applications. In order to enhance the sensor sensitivity, a high electrostatic capacitance under a constant voltage must be achieved, and for this purpose, a high dielectric constant is required in the material that is used. Examples of a polymeric material with high specific permittivity include polyvinylidene fluorides and polyvinylidene fluoride copolymers, which are known to be used as piezoelectric materials or pyroelectric materials. For example, Japanese Unexamined Patent Application Publication No. 2010-108490 (Patent Literature 1) describes a transparent piezoelectric sheet including a vinylidene fluoride-tetrafluoroethylene copolymer, and use of the sheet in a touch panel. Furthermore, Japanese Unexamined Patent Application Publication No. 2011-222679 (Patent Literature 2) describes using a vinylidene fluoride-trifluoroethylene copolymer and polyvinylidene fluoride as a material for a transparent piezoelectric body film of a transparent piezoelectric sheet, in addition to a vinylidene fluoride-tetrafluoroethylene copolymer. On the other hand, a vinylidene fluoride polymer has high crystallinity, and therefore has problems with inferior processability for manufacturing a molded product such as a film or the like.
On the other hand, the specific permittivity of a general-purpose polymeric material with excellent transparency and processability normally has a value of 2 to 4, but by appropriately designing the polymer structure, the value can be increased to 5 to 7 or higher. In order to enhance the specific permittivity of polysiloxane, introducing a fluoroalkyl group as a substitution group on a silicon atom was already known to be effective, and the present inventors also disclosed that an organopolysiloxane cured product containing a fluoroalkyl group has high specific permittivity, and is useful as a transducer material (International Patent Publication 2014-105959, Patent Literature 3).
On the other hand, a pressure-sensitive adhesive containing an organopolysiloxane containing a fluoroalkyl group is also already being researched. For example, U.S. Pat. No. 5,436,303 (Patent Literature 4) discloses a pressure-sensitive adhesive containing a fluoroalkyl silicone and MQ resin having a silanol group, formed from R3SiO1/2 units (where R represents an alkyl group) and SiO4/2 units. Furthermore, International Patent Publication 1994-6878 (Patent Literature 5) describes a copolymer formed from a fluoroalkyl silicone and vinyl monomer and a pressure-sensitive adhesive with favorable solvent resistance containing the copolymer. With these technologies, an MQ resin which is a key for pressure-sensitive adhesion does not have a fluoroalkyl group, and the transparency is unclear and actually not described. On the other hand, U.S. Pat. No. 7,253,238 (Patent Literature 6) describes an MQ resin containing a fluoroalkyl group, formed from R3SiO1/2 units (where R represents an alkyl group), R(CH2═CH)SiO2/2 units (where R represents an alkyl group), and SiO4/2 units, and a pressure-sensitive adhesive containing the resin. Furthermore, U.S. Pat. No. 5,066,078 (Patent Literature 7) discloses an MR resin containing a fluoroalkyl group where the method of manufacturing is limited, and a pressure-sensitive adhesive containing the resin. Furthermore, Japanese Unexamined Patent Application Publication No. 2010-502781 (Patent Literature 8) also discloses a laminate body formed from a fluoroalkyl silicone pressure-sensitive adhesive composition and silicone liner. However, with these pressure-sensitive adhesive compositions, only a peroxide curing system is disclosed, and an addition type curing system with a high curing rate, almost no shrinking during mold processing, and in which desired curing conditions are easy to set has not been studied. Furthermore, dielectric properties also have not been studied, and there is no mention nor suggestion thereof. Therefore, an addition curing type organopolysiloxane containing an fluoroalkyl group with excellent transparency and high specific permittivity, a pressure-sensitive adhesive laminate body film containing the organopolysiloxane, and a display device configured from the pressure-sensitive adhesive laminate body film have not been reported to date.
On the other hand, an optically transparent silicone pressure-sensitive adhesive film and a structure of a display device such as a touch panel or the like using the film is disclosed in Japanese PCT Patent Application 2014-522436 (Patent Literature 9), Japanese PCT Patent Application 2013-512326 (Patent Literature 10), and the like, but the performance of the silicone pressure-sensitive adhesive film still has room for improvement.